We recognize that it is desirable to construct apparatus for digging and transplanting trees, commonly and hereinafter referred to as “tree spades”, having the lowest possible overall height, consistent with effective digging power, since the vehicle-mounted apparatus must manoeuvre through orchards and facilities in which the frame-mounted towers carrying the extended sliding spade assemblies should avoid lower tree branches.
In one known example of apparatus for digging and transplanting trees [U.S. Pat. No. 5,459,952 (Tillaart et al.)], a multi-sided or circular frame for encircling a tree to be transplanted has mounted thereto a plurality of angularly upwardly extending towers, each including a sliding spade assembly. Each sliding spade assembly is pulled down the tower in the digging stroke, by powered retraction of hydraulic cylinders coaxially affixed to each incorporated by reference for its description and illustrations of example of tree spade apparatus of a kind in which a hydraulic cylinder assembly according to the present invention is advantageous.
A number of designs for multiple-stage hydraulic cylinder and piston assemblies are known for use as actuators in a wide variety of lifting, digging and hammering apparatus. Telescopic constructions of cylinder and piston are adapted to collapse when retracted to a short configuration and to be extended to secure a desired length of travel. Examples are afforded by U.S. Pat. No. 1,095,926 (Powell) and U.S. Pat. No. 5,322,004.
A number of “double-acting” telescopic cylinder assemblies have been devised for a range of applications. In assemblies of this kind, fluid pressure is applied to each of the telescopically associated parts in either direction of movement. One such cylinder assembly is described in U.S. Pat. No. 2,691,963 (Seng), intended for installation in bulldozers, cranes, power shovels and the like. U.S. Pat. No. 5,613,418 (Guido) describes a three-stage hydraulic lifting cylinder in which the first stage is in the form of a hollow-piston cylinder with a cylinder tube, a piston and a piston rod. The piston rod is hollow and accommodates a telescoping cylinder in the form of two long-stroke cylinders. The hollow piston rod and the telescoping cylinders include stops for limiting the stroke travelled by the two telescoping cylinders.
Telescoping hydraulic cylinders are generally constructed with the aim of obtaining a large elongation in comparison with the collapsed or retracted length of the assembly. For this reason, they are not ideal for actuating the digging blades of a tree transplanting apparatus, in which the digging stroke involves retraction of the cylinder. For that application it would be desirable to limit the extended height of the blade assemblies as much as possible for the given digging stroke distance.
Moreover, with conventional double-acting multi-stage telescoping cylinders, the high back pressures encountered lead to energy inefficiency and potential mechanical problems if used to actuate tree transplanter apparatus.